1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer, and more particularly to an image forming apparatus including a write head that writes dots in accordance with halftone data.
2. Description of the Related Art
Exposing units for conventional electrophotographic printers are generally of two types: a scanning type head with a light source such as a laser and an LED head having a large number of light emitting diodes (LEDs) aligned in line. A laser head is capable of changing the size of dots from dot to dot in the same scanning line. In contrast, an LED head is usually not capable of readily changing the size of dots from dot to dot in the same scanning line. In order to overcome the drawback of an LED head, it has been proposed to form a line of halftone pixels by combining sub dots formed in a plurality of sub-lines. Dots on the same sub line have the same exposure energy and dots on different sub lines in the same pixel have different exposure energies. Combining dots having different sizes on sub lines implements a desired halftone pixel.
A tandem type color printer incorporates a plurality of LED heads, each LED head illuminating the charged surface of a corresponding photoconductive drum to form an electrostatic latent image of a corresponding color. The electrostatic latent image is then developed with a toner of the corresponding color. Images of the respective colors are transferred in sequence onto a piece of paper one over the other in registration to form a full color toner image. The LED heads may be mounted with some positional errors, so that the line of LEDs extends somewhat obliquely with respect to the rotational axis of the photoconductive drum. Thus, a tandem type color printer may suffer from a problem in that when the toner images of the respective colors are transferred onto the paper one over the other, the toner images are shifted. Japanese patent application Laid-Open JP11-34399A discloses a technique for minimizing misregistration of images of respective colors in an advance direction (direction of travel of the paper). Data received from a host apparatus is temporarily stored in an image memory. Data for one line is read from the image memory such that the data is a combination of data from a plurality of lines in accordance with the positional errors of the LED heads. Then, the data read from the image memory is transmitted to the LED head on a line-by-line basis. In this manner, misregistration of the printed images in the traversing direction is minimized.
FIG. 32 illustrates dots printed on paper in conventional halftoning without correction when an LED head is misaligned so that the LED head extends at an angle with a rotational axis of a photoconductive drum. Printing was performed without positional correction in the advance direction by a conventional halftone printing technique. Each dot or pixel is formed of three sub-dots rendered at different halftone levels. The LEDs in the LED head are energized at three consecutive timings to form three sub lines, such that dots on the same sub line have the same exposure energy and dots on different sub lines in the same pixel have different exposure energies.
Referring to FIG. 32, the positions of sub dots in the advance direction are off the line on which the sub dots should be, due to the fact that the LED head is not accurately perpendicular to the advance direction of paper. Because each pixel is formed of three different sub-dots, the position of each pixel requires correction of the positions of three sub-dots.
FIG. 33 illustrates one way of correcting the positional errors of sub dots in which the positions of sub-dots indicated by dotted lines are corrected by 3 sub-lines. Because the correction is coarse, the resulting sub-dots still deviate from where they should be. This correction is insufficient to reduce color shift when a plurality of images of different colors are superposed.